This invention relates generally to processes and apparatus for producing glass foils, and more particularly to a process and apparatus for producing a glass foil using a gaseous blowing agent wherein a glass melt is continuously drawn from a heated crucible of a smelting furnace.
Processes and devices are known wherein glass is heated in a smelting furnace until it reaches its melting point, the molten glass divided into jets of glass by circularly distributed discharge apertures, and the jets of glass combined into a tubularly formed body which can then be drawn-out to a thin cylindrical foil. Processes and devices are also known wherein a glass tube drawn from a stock of molten glass is inflated by a gaseous medium and then pressed together and split on both sides. After being split on both sides, the glass cylinder is wound onto cylinders or reduced to glass flakes.
The glass foils or flakes produced in accordance with the known processes and devices are suitable for reinforcing or strengthening plastics, rubber, asphalt, Portland cement and the like. However, such glass foils or flakes are suitable only to a limited degree for insulation that is subjected to high electrical stresses. Such insulation requires glass foils or flakes that are entirely flat and free from the curls and twists produced during the manufacture of glass foils according to such known processes and devices as a result of local concentrations of heat and fluctuations in wall thicknesses which cannot always be avoided, especially when due to the influence of the outside atmosphere.
Therefore, it is an object of the present invention to provide a process for the production of glass foils, as well as to provide an apparatus for the practical application of such a process which will allow glass foils to be drawn from a mass of glass such that the glass foils have walls of uniform thickness, are entirely straight, and are free of curls and twists.
According to a preferred embodiment of the present invention, a glass melt is heated in a smelting furnace until the glass reaches its melting point. The glass is then conveyed through distribution reservoir to a plurality of annularly arranged distributing slots. The glass flows through the distributing slots and emerges from the slots in a plurality of jets of glass. The jets of glass are then combined at a funnel-shaped wall to form a hollow, cylindrical mass of glass or glass cylinder. The glass cylinder flows downward due to its own weight through heating and cooling zones which are under pressure and gas-heated. The glass cylinder is cooled for shaping and is grasped by circumferentially arranged auxiliary conveyor belts. The glass cylinder is then stretched by the auxiliary conveyor belts while the glass is still in its plastic state to a pre-determined wall thickness. The hollow cylinder is then separated at one side with a separating device and the glass bent so as to form an essentially flat or planar mass of glass. The planar mass of glass is then conveyed into a hot bath of liquids. A draw-off guide roller arranged in the bath then draws-off glass foil in full width. The glass fil is then conveyed from the bath through an after-treatment zone in which the glass foil is treated to a coiling device which coils the treated glass foil.
Apparatus according to a preferred embodiment of the present invention includes a revolving crucible which is electrically heated. The crucible has a pre-melting reservoir, a main-melting reservoir connected to the pre-melting reservoir, and a distribution reservoir connected to the main-melting reservoir. Connected to the distribution reservoir is an annular distributing disk having a plurality of flow-through passages, preferably distributing slots, which open onto a funnel-shaped wall. Heating and cooling zones are included and disposed about the hollow glass cylinder drawn from the crucible so as to form a gas cushion about the glass cylinder. Adjacent to the heating and cooling zones are auxiliary conveyor belts which are used to stretch the glass cylinder to a pre-determined wall thickness. A packing/distance tube is included to seal the apparatus from the outside atmosphere. A bath is also included, as is a seal which is coaxial with the heating and cooling zones and which extends up to the bath. A separating device is arranged between the seal and the bath, and a draw-off guide roller is arranged in the bath. An after-treatment zone is arranged outside the bath as is a device for coiling the glass foil after treatment.
The present invention provides for precise, predetermined temperature control of the individual heating and cooling zones and of the bath of liquid. Such control permits the heated glass cylinder to be uniformly cooled and permits the manufacture of thin and flat glass foils which are free of curls and twists.